XMPP is like email, a very open standard that was designed for interoperability even with more closed servers that included proprietary features and extensions. You can message anyone by email no matter what’s or where’s their server and can be configured to be secure and private. Here a quick overview of the architecture.

XMPP is the only solution that treats messaging and video like email: just provide an address and the servers and clients will cooperate with each other in order to maintain a conversation. Everything else is just an attempt at yet another vendor lock-in.